Arne Næss

Arne Næss, who has died aged 96, was Norway's best-known philosopher, whose concept of deep ecology enriched and divided the environmental movement. A keen mountaineer, for a quarter of his life he lived in an isolated hut high in the Hallingskarvet mountains in southern Norway.

Through his books and lectures in many countries, Næss taught that ecology should not be concerned with man's place in nature but with every part of nature on an equal basis, because the natural order has intrinsic value that transcends human values. Indeed, humans could only attain "realisation of the Self" as part of an entire ecosphere. He urged the green movement to "not only protect the planet for the sake of humans, but also, for the sake of the planet itself, to keep ecosystems healthy for their own sake".

Shallow ecology, he believed, meant thinking the big ecological problems could be resolved within an industrial, capitalist society. Deep meant asking deeper questions and understanding that society itself has caused the Earth-threatening ecological crisis. His concept, grounded in the teachings of Spinoza, Gandhi and Buddha, entered the mainstream green movement in the 1980s and was later elaborated by George Sessions in Deep Ecology for the Twenty-first Century (1995).

Deep ecology teaches that belief in an objective comprehension of nature is belief in a flat world seen from above, without depth, and that such cool, disembodied detachment is an illusion, and a primary cause of our destructive relation to the land.

Næss was also an activist, inspired by Rachel Carson's 1962 book Silent Spring. In 1970, together with a large number of demonstrators, he chained himself to rocks in front of Mardalsfossen, a waterfall in a Norwegian fjord, and refused to descend until plans to build a dam were dropped. The demonstrators were carried away by police but the action was a success. He was the first chairman of Greenpeace Norway when it was founded in 1988 and was also a Green party candidate.

As a mountaineer, Næss led the first expedition to conquer the 7,708m (25,289ft) Tirich Mir, in Pakistan, in 1950. He led a second Norwegian expedition up the mountain in 1964. Mountains were at the centre of his vision and he often asked audiences to practise the Taoist injuction to "listen with the third ear" and "think like a mountain".

In its first form his philosophy was known as ecosophy T - the T standing for the Tvergastein mountain hut where he lived and worked. It was as a teenager on a mountain that Næss met a Norwegian judge who advised him to read Spinoza, the 17th-century Jewish philosopher who taught that God is present throughout nature.

Born in Oslo, Næss earned his doctorate at the city's university and, at the age of 27, became its youngest professor. He continued to teach until 1970. Over the years he published more than 30 books as well as numerous essays and articles.

He faced controversy when deep ecology was attacked as "eco-la-la" by Murray Bookchin, who had founded the social ecology movement in Vermont, US. Bookchin claimed the philosophy came mainly from white, male academics and their students, and that its concerns were akin to New Age occultism, with undertones of paganism, and redolent of quasi-fascist Aryan movements.

Næss did not feel the need to confront the social ecologists, but his movement faced embarrassment at the other extreme when activists of Earth First used its concepts to justify violent action, green Luddism, and a campaign to enforce sterilisation and end food aid to developing nations.

Næss countered that his movement for widening compassion towards non-humans did not imply diminishing compassion towards humans. "We don't say that every living being has the same value as a human, but that it has an intrinsic value which is not quantifiable. It is not equal or unequal. It has a right to live and blossom. I may kill a mosquito if it is on the face of my baby but I will never say I have a higher right to life than a mosquito."

His closest friend in Britain, Stephan Harding, the head of holistic science at Schumacher college, in Dartington, Devon, where Næss conducted courses, said Næss was horrified by suggestions of enforced sterilisation and that droughts and famines were good. Harding argued that Næss accepted that "since we are humans, we have to put humans first. He was against violence."

Næss never managed to translate his awareness of overpopulation into a scheme of practical action. He maintained that a world population of 100 million - roughly a 60th of the present figure - would be compatible with quality of life, but 11 or 12 billion - the level predicted for the end of the next century - would not. He said: "I am, to the astonishment of certain journalists, an optimist. But then, I add, I am an optimist about the 22nd century. And they say, 'Oh, you mean the 21st ...' 'No, the 22nd century.' I think that in the 21st century, we have to go through very bad times and it will hurt even rich countries ... So, I am a short-range pessimist, long-range optimist."

Næss was appreciated, even in old age, for his exuberant, frolicsome manner, which reminded people of Gandhi or the Dalai Lama. He believed awareness of deep ecology was present in us all, especially in childhood, when a butterfly could be regarded as a brother or sister. Like Wordsworth, he lamented the attenuation of such awareness in later life through loss of contact with animals, plants and significant places.

He was knighted by King Harald in 2005 and made a commander with star of the Royal Norwegian order of St Olav First Class.

His nephew was the mountaineer and businessman Arne Næss Jr, the husband of Diana Ross, who was killed in a climbing accident in South Africa in 2004.

Næss was married twice, first to Else, with whom he had two children. She predeceased him. He later married Kit Fai, a Chinese student four decades his junior, whom he met when he was 61. She survives him, along with his children.